Recently, pollen has been produced in large amounts by excessive tree planting of Japanese cedars and Japanese cypresses. Moreover, industrialization has increased air pollution or the amount of chemical substances released, so that pollenosis, which is an allergic disease due to combined pollution of the chemical substances and the pollen, has been widely spread. Against such an allergic disease, masks are worn. However, the wearing of masks is only symptomatic therapy, and it is impossible to prevent infection through eyes which can not be covered with masks.
On the other hand, stevioside and rebaudioside, primarily contained in leaves of Stevia, have a strong sweet taste in small amounts, and are used as natural sweeteners low in calories in substitution for sugar. Stevia is therefore known as a raw material for natural sweeteners low in calories.
As oral drugs antagonistic against histamine, drugs containing antihistaminic agents such as diphenhydramine hydrochloride and promethazine are commercially available. However, the antihistaminic agents generally have the side effect that they cause drowsiness, which results in a decisive disadvantage when patients tackle work and study enthusiastically.
It is desirable to have substances of natural origin for relieving symptoms of allergic diseases, which have no side effect such as drowsiness and can be continuously given without anxiety of a side effect.